1. Field of Invention
The invention relates to digital attenuators for radio frequency signals and more particularly to electronically reconfigurable pad attenuators in which a plurality of segmented gate field effect transistors are selectively controlled to provide a set of discrete attenuation values.
2. Prior Art
A major need for a reconfigurable attenuator is in low DC power consumption microwave and millimeter wave subsystems for setting nominal gain. In some applications the nominal insertion phase is also critical, and so to minimize calibration complexity, it is highly desirable to have an attenuator whose insertion phase is independent of the attenuation setting. In receivers it is important that the attenuator does not degrade the dynamic range.
A further important application is to provide an extension to the dynamic range of a communications receiver. Under large signal conditions it is desirable to attenuate the signal incident on the latter stages of the receiver to prevent intermodulation products which would result from non-linear operation and interfere with the detection and analysis process. Receivers requiring an extended dynamic range are used in radar systems, electronic warfare systems and communication systems.
A further need for an attenuator having wideband properties is in creating a wideband vector modulator. For this application two such attenuators may be used to adjust the magnitudes of the in-phase and quadrature vectors of a signal which, when recombined, produce a highly accurate phase shift with amplitude control. Such an application is described in Y. C. Hwang et al U.S. Pat. No. 4,638,190 , assigned to the assignee of the present application and entitled "DIGITALLY CONTROLLED WIDEBAND PHASE SHIFTER". A wide band vector modulator requires a pair of wide band attenuators.
Three approaches to gain control are commonly used. One known approach utilizes variable gain amplifiers, either with analog bias control or with digital control of a segmented dual gate FET (SDGFET) device. The second approach utilizes conventional attenuators which are of an analog nature. The third approach cascades a series of two state attenuators to create a digital multistate attenuation.
The disadvantage in using a variable gain amplifier is its DC power consumption, typically several hundred milliwatts. These amplifiers have the further limitation of experiencing degradation in RF output power, noise figure and dynamic range when the gain is varied. Many variable gain amplifiers exhibit a large variation in the insertion phase as the gain is adjusted.
Conventional analog FET attenuators have a non-linear relationship between the control voltages and the attenuation characteristic when well matched performance is required. Linearization of this relationship may be achieved with off-chip silicon circuitry or on-chip gate balancing resistors coupled to a segmented gate FET, both methods consuming significant power. Linearization is a pre-requisite for digital control. The cost of implementing these approaches is increased because of the manufacturing variations between attenuators.
The cascade of two state attenuators typically has a high insertion loss and is inconveniently large to be realized as a low cost MMIC. This approach can also suffer from long settling times.